It has been known that when an AgX emulsion which contains tabular grains having a large aspect ratio (diameter/thickness) is coated on a support and used as a photographic material, in particular, when the tabular grain emulsion is used in an upper layer, scattering of light to a lower layer is reduced and the sharpness can be improved. Further, it has been known that a large amount of spectral sensitizing dyes can be adsorbed because the tabular grains have a larger surface/volume ratio, and a light absorption factor and a sensitivity granularity ratio are improved. Accordingly, the tabular grains have conventionally been used in various photographic materials. However, there is a harmful effect such that when the tabular grains are produced by conventional methods, the higher the aspect ratio, the higher becomes the degree of polydispersion, therefore, an optimal chemical sensitization cannot be carried out.
Various technical examinations have been made with a view to improving this drawback. For example, techniques for monodispersion of tabular grains are disclosed in JP-A-52-153428 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-55-142329 and JP-A-61-112142, but these techniques are insufficient in the effect of monodispersion.
Moreover, there are disclosed in JP-A-63-151618, JP-A-1-158426 and JP-A-2-838 that monodisperse tabular emulsions can be obtained by using low molecular weight gelatin at the time of nucleus generation.
In addition, methods for producing monodisperse tabular grains in the presence of a polyalkylene oxide block copolymer at the time of nucleation are disclosed in U.S. Pat. Nos. 5,147,771, 5,171,659, 5,147,772 and 5,147,773. A monodisperse tabular grain emulsion having a variation coefficient of 10% or less is disclosed in EP-A-514742. However, the above polyalkylene oxide block copolymers are used in the execution of the working examples of this patent.
It was confirmed that monodisperse tabular grains could certainly be produced by the techniques as disclosed in the above patents, but the monodispersibilities of these grain size distributions were not sufficient, in particular, the graininess was not satisfactory.
Further, when the halide composition of a silver halide emulsion is not uniform, for example, silver iodobromide or silver chloroiodobromide is used, even if the shape of the grains is monodisperse tabular grains as above, that alone will not necessarily be sufficiently effective to increase high contrast of gradation and to reduce unevenness of chemical sensitization among grains as expected above and further improvements of gradation, development progressing capability, pressure capability and preservability have been desired.